Building an amplifier
(WALL OF TEXT WARNING!)
My housemate's PC power supply recently gave up in a cloud of smoke. Post-mortem revealed a blown rectifier and HV diode packs, shorted 3.3v rail transformer and a few bulging capacitors. Not worth repairing as it was only a cheap thing...
However, after stripping the PCB for parts, I was left with a rather nice case. Half extruded aluminum, half clear polycarbonate which fit together with a rather nice tongue and groove, held in shape by machine screws through the end panels. Remarkably well made for a shitty PSU.
Now...I've been wanting to build a Gainclone amp for some time as it's relatively cheap and easy. (and I have a couple weeks off work. drunkeness + projects shall happen, although not at the same time) They're based around an LM3875 series amp-on-chip made by National Semiconductor, which is remarkably hi-fi for the price and doesn't need many external components. Originally, a $3k amp called the Gaincard was based around these $12 chips until someone opened it up and found out that despite the low price of the parts, it sounded pretty decent and made a copy, hence GainClone.
There's a million and one circuits out there, each of which claim to be the best. I'm going to be using the recommended circuit off the datasheet with a few modifications to the feedback loop and input filter.
It's going to be a very tight build in that small box so there's no room for big passive heatsinks. I'm therefore going to use the PSU's original fan in the back, running off 7v and reversed so it blows against a heatsink from an Xbox 360's GPU as I've managed to get hold of a bunch for cheap.
Again, to maximise space I'm going to use an EI core transformer as opposed to a toroidal one. They're the ones shaped like donuts...better for audio as there's less field leakage but in this case it's not as economical with space. Everything will be built on copper stripboard as point-to-point wiring is only sexy on tube amps, and I don't want to get a PCB etched. All the connectors apart from power are going to have to be on the side as there's no space on the back...yep, it's tight alright. Avoiding crosstalk and feedback caused by crosstalk between wires is pretty difficult when everything is crammed in. I'm going to run the signal lines down one side and power down the other, and keep signal ground seperate to power ground.
The power amp IC is going to be used in a non-inverted configuration so I can easily set the input impedance to suit the preamp using a potential divider on the input. A potentiometer (variable resistor) is the easiest way of doing this and it allows the input gain to be changed easily.
Most people use the Gainclone with a passive preamp and impedance matched sources so the sound is as transparent as possible for use with high quality fullrange speakers. However, I want this thing to be as flexible as possible to drive different speakers and accept input from many different sources at the expense of transparency. A preamp and impedance buffer is therefore needed to make sure the power amp sees the same impedance whatever the source and for tone controls.
Now, if I was building it in three seperate chassis...one for PSU, preamp and power amp, I'd make a traditional opamp based pre. However, since it's all crammed into one box, a normal pre would suffer from all the EMI and electrical noise floating around in there. Philips make a nice little integrated preamp in one IC (TDA1524A) which doesn't need many outboard components which means I can cram it into a metal box for shielding. This IC has a fairly low output impedance which is a Good Thing, so the power amp's input impedance can be set to about 10k.
[When matching line level components, you always want to have a low output impedance driving something with a high input impedance to avoid low end rolloff and muffled treble.]
Schematics to come in the next post. Here's the power supply snubber circuit I'm using which is widely used and appreciated, created by Carlos Filipe of DIYAudio:
(upload to come when I get real internet back, right now I'm using my phone as a 3G modem)
So far I've ordered all the preamp and power supply parts, minus the transformer. Oh, and I've bought two LM3886 ICs, the insulated version so I don't need to insulate it from the metal heatsink with mica sheets.
So yeah, this is going to be a log over the next couple weeks for anyone who's interested. Any tips would be welcome too.
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